74 TRANSFORMATIONS OF ROCK-MATTER. 



matter here baking and hardening, there calcining and 

 melting ; here inducing crystallisation, and there, in com- 

 bination with other forces, filling veins with veinstuffs and 

 metallic ores. 



In connection with this deep-seated heat, we may also 

 believe that magnetic currents and crystallisation (to what- 

 ever force or forces it may be due) are also effecting a 

 marked metamorphism on all the older and deeper-seated 

 rocks. The earth is in fact a great magnet, through whose 

 exterior crust currents of varying intensity are ever passing ; 

 and these, we may rest assured, are actively instrumental in 

 altering the molecular arrangement of rock-masses, and con- 

 ferring upon them not only new textures, but also new 

 structures, such as cleavage, crumpling, foliation, and other 

 peculiarities which distinguish the slates and schists of the 

 older formations. Experiment has tried to imitate this 

 mode of metamorphism by passing galvanic currents through 

 masses of moistened pottery clay, and the result was re- 

 arrangement of the particles so as to produce cleavage, or 

 fissility, such as occurs in roofing-slate, and at the same 

 time a glistening and semi-crystalline texture. Taken in 

 connection with heat, these subtle forces of chemistry, 

 magnetism, and crystallisation, are obviously important 

 modifiers of mineral and metallic matters ; and thus, among 

 the oldest rocks, which have been the longest subjected to 

 their influence, we find crystallisation, cleavage, foliation, 

 and kindred phenomena in their greatest intensity. It is 

 to these older rocks that the term METAMORPHIC is gene- 

 rally applied ; and though all rock-matters are continually 

 undergoing metamorphism, and in some localities intensely 

 so, yet these old semi -crystalline and highly -altered slates 

 and schists are so intimately associated in character and 

 position that they have been grouped into a system the 

 " METAMORPHIC SYSTEM " of Systematic Geology. 



